CD4 T helper (TH) cells drive autoimmunity through their production of proinflammatory cytokines. The molecular determinants controlling the cytokine production and pathogenicity of autoreactive T cells remain unclear. In experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis (MS), TH1, TH17 infiltrate the central nervous system (CNS). Remarkably, IFN-? and IL-17, the hallmark cytokines produced by TH1 and TH17 lineages, are not required for encephalitogenicity. Instead, the TH cell- cell-derived cytokine granulocyte-macrophage colony stimulating factor (GM-CSF, encoded by Csf2) plays a nonredundant role in mediating neuroinflammation. Immunosuppressive cytokines produced by autoreactive T cells, including IL-10, also influence T cell encephalitogenicity. We have discovered that mice deficient for the transcription factor Bhlhe40 are resistant to the induction of EAE. Bhlhe40-/- TH cells produce normal amounts of their hallmark cytokines, but produce decreased amounts of GM-CSF and increased amounts of IL-10. We will test the central hypothesis that Bhlhe40 is a required determinant for the pathogenicity of autoreactive T cells. In Aim 1, we will determine the mechanistic basis for Bhlhe40 function in neuroinflammation, testing the hypothesis that the nonencephalitogenicity of Bhlhe40-/- T cells is due to their cell-intrinsic IL-10 production. We will also test the hypothesi that Bhlhe40 acts as a direct transcriptional regulator of genes controlling autoreactive TH cell pathogenicity and determine the structural features of the Bhlhe40 protein required for this transcriptional regulation. In Aim 2, we will use Bhlhe40-green fluorescent protein (GFP) reporter mice to identify the signals that induce Bhlhe40 expression in T cells and test the hypothesis that Bhlhe40 expression determines the pathogenicity of autoreactive T cells. These studies will contribute to our understanding of how T cells acquire autoaggressive effector functions and could identify Bhlhe40 as a novel therapeutic target for the treatment of autoimmune disease.